•208 ALASKAN GLACIER STUDIES
cannot be carried out of the kettles into which they slide, we assume that kettle areas of one period become hummock crests in others, and that one-time ridges later develop into kettles. If this is so, the position of the glacial drainage is constantly changing in detail.
By the ablation and the consequent drainage some of the moraine is carried off by the streams, but the proportion thus removed cannot be great, otherwise it would be impossible for such extensive sheets of ablation moraine to accumulate. Most of the rills are fairly clear and free from sediment, and since the streams do not ordinarily .attain large size or flow for more than a few score yards before descending into the glacier, the load which they bear cannot be great. They certainly could not carry bowlders and large pebbles, and the evidence is convincing that they do not carry large quantities of even the finer material. The glacial torrents which emerge from the margins of the piedmont bulbs are, however, heavily burdened with sediment; and, although some is doubtless contributed from the ablation moraine, we infer that much the greater proportion is derived from the lower layers of the glacier.
The extensive sheets of ablation moraine on the piedmont bulbs have two important •effects. In the first place they prevent most of the local influence on climate which such •extensive ice sheets would normally exert; and consequently dense vegetation can grow up to the glacier margin, and even on its surface where soil stability is sufficient. Sec-•condly, they' retard the melting of the glaciers, especially the outer parts, so that such ice sheets persist long after they would if not moraine covered. It would be interesting to know the rate of lowering of such covered glaciers, and it is our hope and expectation to undertake some such measurement, or at least to provide a beginning for measurement. Until, however, we have some carefully-run lines of levels, and some knowledge •of ground temperature on and near the buried ice masses there can be little gained from .a discussion of the probable duration of the buried stagnant ice. That it melts very .slowly and lasts a long time is abundantly proved.
Moraine-covered glaciers are not confined to Alaska, though it is from the Alaskan glaciers that we have obtained out fullest knowledge of this condition. They are present in the Himalayas, and there are ice tongues resembling them, though without piedmont bulbs, in the Alps. In both these cases, and, in fact, in general, and even in Alaska, "the clear ice glacier is the normal, and the moraine-covered glacier the exception. Our knowledge of the distribution of this type of glacier and of the surrounding conditions is not sufficient for anything like a final consideration of its cause. Yet there are some facts already observed in the Alaskan region which contribute toward an explanation of the phenomenon. Among these one of the most notable is the fact that the larger glaciers are less liable to the condition than the smaller; and another noteworthy fact is that small, valley-enclosed glaciers are commonly moraine-covered whereas the large through glaciers are not. A third noteworthy fact is that the ablation moraine is prevailingly made of angular fragments such as are contributed by avalanches. The inference that we draw from these three facts is that hi certain glaciers whose walls are steep enough, whose width is small enough, and whose enclosure is sufficiently complete, •enough d£bris is avalanched into the reservoir and out upon the valley tongues to provide material with which ultimately, through ablation, to clothe the entire glacier surface with a sheet of ablation moraine. In broad glaciers and in through glaciers all the necessary conditions are not present and consequently complete covering by ablation moraine becomes impossible.